The present invention relates to the field of flexible pipelines used in petroleum exploration. These pipelines are used on the one hand to provide connections between the sea bed where the well head is located and the surface where an oil platform is generally installed in order to treat and dispatch production. They are generally used on the other hand to convey the effluent produced by wells, in the form of liquid or gaseous products, between a storage or processing site and the point at which it will be used. These flexible pipelines therefore convey the petroleum output and any products which might be associated with it (liquid crude oil and/or gas, under pressure and at high temperature, as well as various other fluids such as oil, methanol, . . . ).
The general structure of flexible pipelines is specifically described in document API RP 17B (First Edition Jun. 1, 1988). The flexible tubular pipelines are generally made from different elements starting from the axis of the pipe out towards the exterior, including:
a flexible metal tube, referred to as the internal frame, made from at least one section, the turns of which are stapled to one another,
a polymer sealing sheath,
at least one layer of reinforcing forming a pressure-resistant casing due to its small-pitch spiral design,
at least one layer of traction-resistant reinforcing wound spirally at an elongate pitch,
in the case of a flexible pipe which does not have a shell, a layer of reinforcing crossed at an angle of approximately 55xc2x0,
other external or intermediate sealing sheaths.
The specification for polymer sealing sheaths used to cover the frame is generally one which is particularly difficult to meet because it has to provide a seal for the flexible tubes since it will be in direct contact with the products being transported which are under pressure and at high temperature. Essentially, this sheath:
may be made by continuous extrusion, possibly directly onto the internal frame as a base,
must be sufficiently flexible to cope with the curvature imposed on flex-pipes during manufacturing operations and on-site laying (swell movement or lifting of the flexible line in preparation for a move to another site),
must have a high resistance to creep in order to cope with the force of pressure, which is aggravated by the level of the temperature. Creep occurs in the jointing (spacing or clearance) between the metal reinforcement (for example self-locking zeta or T) against which the sheath bears when the pipe is placed under pressure by the effluent conveyed,
must be sufficiently chemically stable to ensure that there is no latent deterioration in its mechanical properties and sealing capacity during the service life of the flex-pipe.
The criteria governing the choice of materials which may be used to make the sheaths, particularly if service temperatures are likely to reach or even exceed 130xc2x0 C., limits the number of possible materials significantly. The various manufacturers generally offer three types of material for these applications.
Polyolefins (polyethylene of average density) are compatible with the specifications listed above but their creep resistance is limited at high temperatures, especially in the presence of liquid hydrocarbons, which are absorbed by these materials to a high degree. The maximum operating temperature for polyethylenes is around 60xc2x0 C. in the absence of gas.
Of the polyamide family, only polyamides 1 and 12 have sufficient chemical resistance for use in these applications. The resistance of polyamides to hydrolysis limits their service life to about twenty years in temperatures of about 90xc2x0 C. in the presence of crude oil and gas. In the presence of water, their useful life is shorter and will then depend on the temperature and acidity conditions. The degree of flexibility demanded of sheaths incorporated in a flexible pipe structure for use in these applications makes it necessary to include a plasticizer product (external plasticization) to make these materials more flexible.
At temperatures in excess of 90xc2x0 C., vinylidene fluoride homopolymers are used for their high chemical inertia. The mechanical behaviour, particularly in terms of creep resistance, restricts their use to temperatures below 130xc2x0 C. As with polyamide homopolymers, the level of flexibility which is vital to cope with the movements imposed on flexible pipelines is such that materials of this type have to be plasticized (external plasticization).
This plasticization, referred to as external, is effected by mechanically mixing the polymer with components with a low molecular mass, which increases the elastic deformation limit of the material. Under certain conditions, however, these low-mass compounds may dissolve in the crude oils which come into contact with the sealing sheath. As they gradually lose their plasticizer, these materials exhibit a lower capacity of elastic deformation and risk becoming too susceptible to cracking when the flex-pipe is subjected to movement.
Manufacturers of flexible pipeline propose systems comprising multiple layers of a same material, for example plasticized double-layer or triple-layer PVDF or plasticized polyamide. The first layer in direct contact with the petroleum is considered as dispensable and may lose its plasticizer and even crack. Since the second layer is not in direct contact with the petroleum, it retains its plasticizer and flexibility and continues to provide sealing functions and mechanical resistance in terms of creep strength. It is clear that the more resistant the sheath has to be to pressure and creep at the joints between the reinforcing, the thicker the superposed layers must be. In certain applications, multiple layers are known which are made up of two different materials such as plasticized PVDF as an internal sheath and plasticized polyamide as an external sheath. However, these systems give rise to problems as regards the compatibility between the different constituent materials, particularly at high temperature.
With multiple-layered sheaths of this design, the two main functions are not dissociated from one another: chemical resistance (resistance to ageing and the loss of plasticizers) and mechanical resistance particularly as regards creep in the spacing between the pressure-resistant reinforcing and other reinforcing.
Surprisingly, it has now been found possible to dissociate the functions of mechanical creep resistance and chemical resistance and to manufacture, at a lower cost, sealing sheaths which meet the specifications for flexible pipelines, even as far as broadening their potential applications, using materials which, individually, do not meet all the requisite specifications. This being the case, both methods are used to plasticize polymer materials, internal plasticization (flexibility obtained by grafting or polymerisation of monomers, making the polymer chain more flexible, but which are still free to migrate) and external plasticization (increased flexibility by a physical addition or admixture of plasticizers which are more or less free, and hence extractible, depending on the environment).
Creep in the jointing spaces between the metal reinforcing (clearance between 0 and approximately 5 mm) represents one of the current limitations on polymer materials for use in a single-material sealing sheath. In fact, it is generally felt that once the volume of substance which has penetrated the jointing space has formed a sub-thickness of 30% of the thickness of the initial sheath, there is a risk that the radius of curvature of the pipeline (blocking) will be limited but also that the build-up of strain will make the plastic sheath susceptible to the onset of cracking. On a sheath with a single layer, these risks are unacceptable and the dimensions of the sheath are chosen accordingly. Choosing the right material for the external layer (specified for mechanical resistance) can specifically overcome this problem.
The present invention, therefore, relates to reinforced flexible piping for petroleum exploration, in which the sealing sheath consists of at least two layers of an extruded thermoplastic material. In the sheath:
the external layer, relative to the axis of the tube, is made from a polymer or copolymers of given mechanical characteristics, possibly plasticized internally or externally;
the layer inside the one described above is made from a polymer or copolymer with no external plasticizer. The polymer or copolymer with no external plasticizer
in the internal layer may be selected from among the group comprising:
the homopolymers or copolymers of ethylene and/or alpha olefins, for example the polyethylenes or polypropylenes;
the polymers of vinylidene fluoride or the copolymers of vinylidene fluoride CF2xe2x95x90CH2 and at least one monomer of the type CF2xe2x95x90CFX, where X may be F, Cl, Br, CF3 to C5F11;
the copolymers of tetrafluoroethylene with monomers of the type CF2xe2x95x90CF_O_R, where R may be CF3(MFA), C2F5, C3F7(PFA), C4F9, C5F11;
the copolymers of ethylene with TFE or CTFE;
the polyketones.
The polymer of the external layer may be selected from the group comprising polyamide 11 or 12 and PVDF.
The polymer of the external layer may have a mechanical strength such that the shear modulus of the polymer may be greater than 30 MPa for temperatures lower than approximately 140xc2x0 C. and the shear modulus may be less than 1000 and preferably less than 700 MPa at temperatures in excess of xe2x88x9220xc2x0 C.
The present invention also relates to the industrial application of the pipeline described above for conveying an effluent, generally petroleum, under pressure and at high temperatures, i.e. which may reach 180xc2x0 C. but by preference not exceeding 130xc2x0 C.
Like all the plastic materials used in this application, elongation at the traction threshold must be at least 5%.
The flexible plastic materials which are not externally plasticized may be of considerable interest for this pipe structure.
Incorporating comonomers in the structure of polymers (internal plasticization) effectively increases their flexibility but limits their mechanical strength, particularly a regards creep, and hence their service temperature. For example, the thermoplastic copolymers of PVDF (polyvinylidene fluoride) in which 2 to 20% by mole of monomers, such as HFP (hexafluoropropylene), CTFE (chlorotrifluoroethylene), BTFE (bromotrifluoroethylene or TFE (tetrafluoroethylene), are introduced into the structure of the base polymer, VDF (vinylidene fluoride). The flexibility of these copolymers is superior to that of the homopolymer VDF but they are mechanically limited as regards service temperature.
These materials which have good chemical resistance at high temperatures and under pressure are suitable for the internal layer which is in contact with the effluent being conveyed by the pipeline.
In the structures proposed by the invention, the external layer is by definition protected from contact with the hydrocarbons or water by the internal chemically resistant layer. Consequently, materials may be used which are optimised in terms of mechanical strength, particularly with regard to creep between the pressure-resistant reinforcing. These materials may be externally plasticized, particularly since migration of the plasticizer may be limited here due to the presence of an internal layer as proposed by the invention.
This type of structure has the advantage of separating the functions of chemical resistance and mechanical strength. In this configuration, chemical resistance over time is provided by the internal layer, which is supported the external layer which in turn provides the mechanical strength.
In manufacturing a dual-layer sealing sheath, the intention is not to join the two layers so that when the pipe is bent, the two layers will have a certain degree of longitudinal freedom relative to one another. As a result, the rigidity of the unit will not be over-increased. In particular, no bonding agent will be used and on the contrary, the exterior of the internal sheath may be coated with anti-bonding products such as talc, lubricant, etc.